Problem: Diabetes is the sixth leading cause of death in the US and is predicted to increase in incidence by 42% from 1995 to 2025. Diabetes also contributes to increased rates of morbidity with overall estimated costs of $174 billion annually. Although most adults with diabetes seek their care from primary care physicians, adherence to diabetes treatment guidelines in these settings remains less than optimal. Practice improvement efforts typically require substantial practice and outside resources thus limiting the potential for translating these improvements into the financially stressed primary care environment. We will address the question: Can primary care practices use a low-cost multifaceted organizational change process to improve adherence to diabetes guidelines? Purpose: To evaluate the effectiveness and cost-effectiveness of a multi-faceted organizational change intervention focused on diabetes registry adoption for improving the quality of diabetes care in primary care. Methods: We will conduct a group-randomized controlled clinical trial in 30 primary care practices of an intervention based on our successful R34 pilot study of diabetes registry implementation. Participating primary care practices will conduct an organizational self-assessment and use findings from this process to direct the implementation and use of a disease registry. Practice leaders will receive brief education on population health approaches to diabetes care as well as limited support for planning work process changes and basic technical support as needed. We will use methods and instruments refined in the R34 pilot to collect patient-level outcomes data and document intervention costs. Outcomes: We will assess diabetes care quality through medical record review at baseline and at regular intervals throughout the project. We will collect intervention cost data through observation and practice logs of intervention activities in order to conduct a cost-effectiveness evaluation of the intervention. A multi-faceted qualitative process evaluation will document the intervention as delivered and provide a detailed understanding of the factors associated with implementation and sustained use of a diabetes patient care registry for dissemination. Benefit: Findings from this study will lead to a better understanding of how to leverage existing practice resources and concerns for improving diabetes care in primary care practices.